The invention provides an implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
A popular mode of operation for dual-chamber pacemakers is the DDD mode. Specifically, DDD systems provide atrial pacing during atrial bradycardia, ventricle pacing during ventricular bradycardia, and atrial and ventricular pacing during combined atrial and ventricular bradycardia or heart block also known as AV block. In addition, DDD systems provide an atrial synchronous (atrial tracking) mode. This enables ventricular activity to track atrial activity to more closely approximate the normal response to exercise, or other physiological activity demanding a faster heart rate, by permitting a rate increase to occur commensurate with the rate of the sensed P waves. This advantageously increases cardiac output and facilitates maintenance of AV synchrony.
Atrial fibrillation is a common atrial tachyarrhythmia. Although it is not life threatening, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. Symptoms of atrial fibrillation may include heart palpitations and dizziness.
Atrial fibrillation can occur suddenly. It results in rapid and chaotic activity of the atria of the heart. The chaotic atrial activity in turn causes the ventricular activity to become rapid and variable. It is an extreme environment in which to properly pace the heart in a tracking mode. Hence, it is particularly important for pacemakers to include mode switching for switching from an atrial tracking mode to an atrial non-tracking mode.
Automatic mode switching (AMS) is intended to limit excessive pacing in the ventricles, due to atrial tachycardia or fibrillation, by switching from an atrial tracking pacing mode to an atrial non-tracking pacing mode. During AMS, the system will pace at a constant rate in the ventricles if it is not inhibited by an intrinsic ventricular sensed event. AMS addresses the problem of potential excessive ventricular pacing at some cost. At the time of a mode switch, the ventricular pacing rate drops to the programmed AMS rate, which is often much lower than the current pacing rate. During an episode of atrial fibrillation, the electrical signals from the atria often become small and highly variable, increasing the incidence of inappropriate sensing with the fixed atrial sensitivity setting previously determined during normal sinus rhythm. Due to the inappropriate atrial sensing, the pacing system can easily fall in and out of the atrial non-tracking mode and atrial-tracking mode. This is a mode switch oscillation. The result is that the patient is alternately paced at higher ventricular rates when the system is out of the atrial non-tracking mode and at a much lower ventricular rate when it is in the non-tracking mode. This increases patient morbidity.
The present invention eliminates mode switch oscillation due to inappropriate sensing. By automatic adjustment of atrial sensitivity to appropriate levels during mode switch, mode switch oscillation is avoided. This reduces the discomfort of patients and improves their quality of life.